I should probably have mentioned that this is Captivate 5.
Have some questions: are those two files that you want to combine in one CP-file, if I understand well, the same resolution? How did you copy the slides? And what is the Quality setting of the slides (Properties panel, General region)?
For the PowerPoint slides, I used the Create New command from the startup screen.
Create New > From Microsoft PowerPoint
I just used let it go with whatever the default settings are.
For the simulation, I also used the Create New command from the startup screen.
Create New > Software Simulation
Screen Area: Custom Size: (I didn't keep the exact dimensions)
Recording Type: Automatic: No Panning: No Narration
Finally, I just copied slides from the simulation and pasted them to the PowerPoint-conversion slideshow. This was a first jab for me, so I'm trying to figure out what the parameters are for the program.
Check the resolution for both files (have a look in Modify, Rescale Project): Width and Height in pixels. I think that both files have a different resolution (dimensions in pixels). Then you could downscale one of them, but if the ratio width/height is not the same, that is a problem. Please can you check?